A video camera typically forms an analog composite video signal which is representative of a moving optical image received by the camera. To form the video signal, a sensing spot moves across an image area according to a series of horizontal scan lines arranged from the top of the image area to the bottom of the image area. A complete scan of the image area represented by the video signal is referred to as a “frame.” When the bottom of the image area is reached, the process begins again at the top thereby forming a series of frames. National Television Standards Committee (NTSC) and Phase Alternate Line (PAL) are two widely utilized standards for composite video signals.
A composite video signal generally includes a luminance component signal and a chrominance (also referred to as “chroma”) component signal. The luminance component signal contains brightness information for the image. Synchronizing pulses included during horizontal and vertical blanking intervals of the luminance component signal synchronize a video decoder to the luminance signal. This allows the video decoder to distinguish each horizontal scan line and to identify the start of each frame. To form the composite video signal, the chrominance component signal is modulated by a high frequency subcarrier and is superimposed over the luminance component signal. A “color burst,” which is a series of eight cycles at the subcarrier frequency, appears in blanking intervals for synchronizing the video decoder to the chrominance component signal. The chrominance component signal contains color information for the image.
A composite video signal formed by a video camera can be communicated to a location remote from the camera where it is received by a video decoder and utilized for further processing, such as for storage and/or display. To form an image for display, an illumination spot is scanned across a display area according to a series of horizontal scan lines arranged from the top of the display area to the bottom. So that images from the composite video signal are appropriately processed, the video decoder must be appropriately conditioned according to the composite video signal. For example, the decoder must be synchronized to the phase and frequency of the luminance and chrominance signals and must appropriately control gain applied to each. In addition, dc restoration must be performed by clamping the signals to appropriate reference levels.
A finite amount of time is required to appropriately condition the video decoder according to a received video signal. Before the video decoder becomes stabilized, it first locks onto synchronization pulses of the video signal, thereby aligning itself horizontally and vertically with received video signal. Then, the video decoder locks onto the chrominance subcarrier, performs dc restoration and adjusts gain levels. Because each of these steps must be performed before the next, this process of conditioning the video decoder is time consuming. Typically, several frames are received by a video decoder before the decoder is conditioned according to the video signal. Additional time can be required prior to displaying or storing a frame while waiting for the start of the frame to occur.
Under certain circumstances, it is desired to receive video signals from a plurality of video cameras positioned at remote locations and to process the video signals at a central location. For example, a video surveillance system typically includes a plurality of video cameras placed at strategic locations of a site under surveillance. Typical sites include dwellings, such as homes and apartment buildings, and commercial or governmental sites, such as manufacturing facilities, banks, museums, offices and retail stores. The video cameras can be placed so as to observe activities occurring in driveways, in parking lots, near doorways, in hallways, at cash registers, in stockrooms, at loading docks and in aisles. The central location can be a security desk where displays are monitored by security personnel and where video images are stored or can more simply be a storage device located in a utility room from which video images can be retrieved, should the need arise.
FIG. 1 illustrates a schematic block diagram of a video surveillance system 100 of the prior art. A plurality of (n) video cameras 102, 104 and 106 are positioned at various locations and each is coupled to provide a video signal to a respective input of a multiplexer 108. A select input of the multiplexer 108 conditions the multiplexer 108 to route a selected one of the video signals from the cameras 102–106 to an output of the multiplexer 108. The video signals formed by each camera 102–106 typically vary from each other in frequency (i.e. horizontal line rate, chrominance subcarrier frequency), phase (i.e. relative position of the beginning of lines and frames, phase of the chrominance subcarrier), amplitude (i.e. peak to peak luminance amplitude, chrominance subcarrier amplitude) and dc offset.
An output of the multiplexer 108 is coupled to a video decoder 110. The video decoder 110 receives a selected video signal from the multiplexer 108, synchronizes its internal circuits to the video signal, controls gain levels, performs dc restoration on the video signal and places the video signal into a format suitable for storage in a storage device 112 and for display by a display device 114. The multiplexer 108 is typically utilized to cycle through the cameras 102–106 in a sequence such that at least one complete frame is received from each camera and stored in the storage device 112 before moving to a next camera in the sequence. In this manner, a series of sequential frames is obtained by each camera 102–106 and stored in the storage device 112 for later retrieval.
A drawback to the surveillance system 100 illustrated in FIG. 1 is that because a finite amount of time is required to re-condition the decoder 110 each time the multiplexer 108 selects a video signal from a different one of the cameras 102–106 in the sequence, the speed at which the system 100 can scan from one camera to the next is limited. Because significant unauthorized or criminal activity can occur in a matter of seconds, it is typically desired to complete an entire cycle of all the cameras 102–106 in less than one second. As the number of cameras is increased, however, the time required for the system 100 to perform a complete cycle of all the cameras can become unacceptably long.
To address this problem, a video surveillance system can include multiple video decoders. FIG. 2 illustrates a schematic block diagram of a video surveillance system 200 of the prior art having multiple video decoders 208–212. As shown in FIG. 2, a plurality of (n) video cameras 202, 204 and 206 are positioned at various locations and are each coupled to provide a video signal to respective one of a plurality of (n) video decoders 208, 210 and 212. Each of the video decoders 208–212 receives a video signal from the corresponding one of the cameras 202–206, synchronizes its internal circuits to the video signal, controls gain levels, performs dc restoration on the video signal and places the video signal into a format suitable for storage and display. The outputs of the video decoders 208–212 are coupled to corresponding inputs of a multiplexer 214. An output of the multiplexer 214 is coupled to an input of a storage device 216. An output of the storage device 216 is coupled to a display device 218.
Because each video camera 202–206 corresponds to a dedicated video decoder 208–212, each video decoder 208–212 remains conditioned to the video signals received from the corresponding one of the cameras 202–206. Accordingly, by appropriately controlling the multiplexer 214, the video surveillance system 200 can be rapidly cycled through all the cameras 202–206 such that the storage device 216 receives frames from each camera 202–206 in less time than would be required by the video surveillance system 100 of FIG. 1.
Due to the functions required to be performed by the video decoders 208–212, the video decoders 208–212 tend to have a relatively high cost. Accordingly, the video surveillance system 200 of FIG. 2 having multiple decoders 208–212 tends to be significantly more expensive in comparison to the system 100 of FIG. 1 which has only one decoder 110. Further, as the number of cameras increases, this cost difference also increases.
Therefore, prior video surveillance systems exhibit a trade-off in that increases in performance are accompanied by significant increases in cost. Therefore, what is needed is a technique for increasing the performance of a video surveillance system without significantly increasing its cost.